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Abstract
The very high cycle fatigue (VHCF) property of TC21 titanium alloy blunt-notched specimens were investigated by using an ultrasonic fatigue test machine with a frequency of 20 kHz. S–N of blunt-notched specimens illustrated a continuous decrease characteristic with a horizontal line over the 105–109 cycle regimes. However, the fatigue life showed a large scatter for blunt-notched specimens. Blunt-notch significantly reduced the fatigue property in the high cycle and very high cycle regimes compared with that of smooth specimens. The crack initiation modes for blunt-notched specimens in the very high cycle regime can be divided into three types: (i) surface initiation, (ii) subsurface with flat facet, and (iii) subsurface with “facet + fine granular area”. The crack initiation mechanism of blunt-notched specimens is discussed in view of the interaction of notch stress gradient distribution and heterogeneous microstructure. Furthermore, the fatigue limit model based on the theory of critical distance (TCD) was modified for the very high cycle regime, and the scatter of the fatigue property of the blunt-notched specimens were well predicted by using this model.
Highlights
Titanium alloys are widely used for aeronautical structures because of their high specific strength, toughness, and damage tolerance [1]
The investigation by Akiniwa [7] and Yang [8] showed that very high cycle fatigue (VHCF) crack can initiate from the subsurface for some high-strength steel and titanium alloy specimens where a fine granular area was observed at the crack initiation site
The smooth and notched specimens still fail beyond 107 cycles, which corresponds to the cycle life of the conventional fatigue limit
Summary
Titanium alloys are widely used for aeronautical structures because of their high specific strength, toughness, and damage tolerance [1]. Due to the very high vibration frequency of aeronautical structures, their fatigue failure occurs in the very high cycle fatigue (VHCF, i.e., more than 107 cycles) regime, and fatigue cracks are inclined to initiate at the subsurface of high strength titanium alloys [2,3]. It is a great concern to investigate the VHCF crack initiation mechanism due to the interaction between the notch stress gradient and microstructure for aeronautical structures applications. Qian [5] and Schwerdt [6] reported that the crack initiation sites of the notched specimens were always at the root of the notch independent of the number of cycles, and the notches remarkably decreased the VHCF property. The fatigue crack initiation mechanism of notched titanium alloys is not well understood in the very high cycle regime
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